"Just for fun" is probably closest, but that's somehow not emphatic enough.
I ride because it gives me a happgasm. I coined that word about a decade ago when I was dating my ex-wife. There's nothing sexual about it, it's just the itchy, giggly sensation you get in your chest when you are in a state of extreme happiness.
The only time riding doesn't give me colossal pleasure is when I'm utterly exhausted, or my butt's too sore from sitting on the seat for too many hours. Because those are frustrating reminders that my spirit is willing, but my $#*t is weak.
I'm at my happiest at 170+bpm with my pulse thundering in my temples and the wind shrieking in my ears. I don't care for racing, and sometimes I don't even care about the scenery--those things just distract me from the sensations and results of my effort. And my effort is probably the only thing that is truly mine. My possessions will eventually break or be sold after my funeral, and my body won't last forever, but my effort will always be mine.
I own & watch all the Tour (every year since '86), Giro & Classics dvds, but I don't give a rat's tail who wins, who's doping, or who's riding for which team; I watch the DVDs over & over again for the same reason a pilot can never look away from an aircraft in the act of landing.
I recently learned that I did so many things more for the sake of how they sounded in cocktail party conversation than for how much pleasure I actually got out of them: paddling my kayak to Catalina Island, riding centuries, motorcycling across the country & back, running marathons. I stopped doing those things as soon as I realized this, but I eventually gravitated back to riding long & hard. And since we are creatures who cannot resist quantifying, measuring & comparing everything (even our own efforts), I now concentrate on sub-five-hour centuries. I learn a little something new every century, and I have a mini-breakthrough at almost every ride.
It's been years since I've crashed badly enough to lose any skin, but I still have nightmares about sliding on my bare hip sometimes when I fall asleep on my side. It's been years since I've had a girlfriend, and I wouldn't mind having one again, but only if she felt the same way about riding. Abnormal perhaps, but normal is for the happygasm-impaired, IMO.